The present invention relates to an ultrasonic flow sensor, and to a method for detecting the reception time of an ultrasonic signal.
Ultrasonic flow sensors are in particular used to measure the volumetric flow, mass flow, or flow velocity of a gaseous or fluid medium flowing through a conduit. A known type of ultrasonic flow sensor includes two ultrasonic converters offset from each other in the flow direction, each of which generates ultrasonic signals and transmits them to the other respective ultrasonic converter. The ultrasonic signals are received by the respective other converters and evaluated by electronics provided. The travel time difference between the signal traveling in the flow direction and the signal traveling counter to the flow direction is a measure for the flow velocity of the fluid. This can be used to calculate the desired measurement quantity, e.g. a volumetric flow or mass flow.
FIG. 1 shows a typical arrangement of an ultrasonic flow sensor equipped with two ultrasonic converters A, B that are situated in a conduit 3 and spaced apart from each other by a distance L. Inside the conduit 3, a fluid 1 flows at a velocity v in the direction of the arrow 2. The measurement path L is inclined at an angle α in relation to the flow direction 2. During a measurement, the ultrasonic converters A, B send each other to ultrasonic signals, which are either decelerated or accelerated depending on the direction of the flow. The travel times of the acoustic signals here are a measure for the flow velocity to be determined.
FIG. 2 shows a very simplified schematic depiction of a converter arrangement with a set of control and evaluation electronics 4 attached to it. For example, the flow sensor can function in accordance with the so-called “sing-around” method. In this case, the reception of an ultrasonic signal A0 or B0 in one of the converters A, B immediately triggers an ultrasonic signal in the opposite direction.
In order to measure the travel time of an ultrasonic signal A0 or B0, it is crucial to be able to clearly and precisely determine the reception time of an ultrasonic signal A0, B0. A method known from the prior art for determining the reception time will be explained below in conjunction with FIG. 3.
FIG. 3 shows the signal course of a single ultrasonic signal A0, B0. The “reception time” of the signal A0, B0 is defined here as the first zero crossing N0 of the signal after the signal amplitude Amp has exceeded a predetermined threshold value SW (the so-called pretrigger level). In the example shown, therefore, the time t0 would be the reception time of the signal. (The reception time of the signal could alternatively also be determined by evaluating the phase of the signal.)
Contamination, drifting, or aging of the ultrasonic converters, or turbulence in the flowing fluid can cause the amplitude of the ultrasonic signals A0, B0 to vary greatly. If the signal amplitude does not vary too greatly, then there is hardly any interference with zero crossing detection because the same zero crossing is always detected as the reception time and the frequency of the signal remains essentially the same. But as soon as the maximum amplitude of the half-wave approaches the range of the threshold value SW before the time t0, then erroneous measurements of the reception time can occur, for example, if the ultrasonic signal exceeds the threshold value at a later time, consequently causing a false zero crossing to be detected as the reception time.
The object of the present invention, therefore, is to improve the measurement precision of an ultrasonic flow sensor that determines the reception time of an ultrasonic signal by means of zero crossing detection.
This object is obtained according to the present invention by means of the defining characteristics disclosed in claim 1 and claim 7. Other embodiments of the present invention are the subject of the dependent claims.
An essential concept of the invention is comprised of determining a piece of information about the amplitude of the ultrasonic signal and adapting the threshold value (pretrigger value) to the amplitude of the ultrasonic signal. This makes it possible, in the event of an altered signal amplitude, to always be able to detect the correct, i.e. same, zero crossing or the correct event as the reception time.
There are various ways to determine a piece of information about the signal amplitude: a first possibility consists of measuring a signal maximum, preferably the maximum amplitude of the ultrasonic signal, by means of a corresponding device. Another possibility consists of rectifying the ultrasonic signal and determining an average value. This average value is also a measure for the signal amplitude and can consequently be used as a reference value for adapting the threshold value. In addition, many other signal evaluation methods are conceivable for obtaining a piece of information about the signal amplitude.
According to a preferred embodiment of the present invention, the receiver unit of the ultrasonic flow sensor includes a device for measuring the maximum amplitude of the ultrasonic signal. The threshold value can thus be adapted to the current maximum signal amplitude. This sharply reduces the occurrence of erroneous measurements.
A preferred embodiment form of the amplitude measurement device includes a first S/H stage (scan and hold component), which is supplied at its input with the ultrasonic signal or with a corresponding converter output signal and stores the maximum value of the signal amplitude, and a subsequent second S/H stage, which adopts and stores the maximum value of the first S/H stage. The maximum amplitude value thus determined can then be used to generate a desired threshold value (pretrigger level).
To this end, the output signal of the second S/H stage is divided by a voltage divider and the partial voltage (=threshold value) is supplied to a comparator. The comparator preferably switches its output when the converter output signal exceeds the threshold value. After this, the zero crossing detection can then be executed.
In order to prevent the threshold value from fluctuating too much, a low-pass filter is preferably provided, which filters the amplitude information or the threshold value information (i.e. the corresponding signal).
Another embodiment form of the receiver unit includes a rectifier that rectifies the converter output signal. The rectified signal can, for example, be integrated by means of an integrator or filtered by means of a low-pass. The integrator output signal or filter output signal, in turn, allows inferences to be drawn about the signal amplitude of the ultrasonic signal, thus making it possible to adapt the threshold value.
Another embodiment form of the receiver unit includes a differentiator that differentiates the converter output signal as well as a subsequent zero crossing detection unit, which detects the times of the maxima of the ultrasonic signal. The maxima can, for example, be stored in an S/H stage and the maximum with the highest value can be determined from among them.
Another embodiment form of the receiver unit includes two lock-in amplifiers in which the converter output signal is amplified using two reference clock signals; the reference clock signals, respectively, have the frequency of the ultrasonic signals and the opposite frequency, e.g. are phase-shifted by pi/2. If the two amplifier output signals thus produced are integrated or are filtered through a low-pass, then it is possible for the resulting signals u0 and upi/2 to be quadratically averaged
            u      0      2        +          u              pi        /        2            2      to determine a value that represents a measure for the signal amplitude, thus permitting adaptation of the threshold value.
For explanation of FIGS. 1-3, the reader is referred to the introduction to the specification.